A regime shift in sediment export from a coastal watershed during a record wet winter, California: Implications for landscape response to hydroclimatic extremes. Issue 12 (19th June 2018)

• Record Type: Journal Article
• Authors: East, Amy E.; Stevens, Andrew W.; Ritchie, Andrew C.; Barnard, Patrick L.; Campbell‐Swarzenski, Pamela; Collins, Brian D.; Conaway, Christopher H.
• Is Part Of: Earth surface processes and landforms. Volume 43:Issue 12(2018)
• Page Start: 2562
• Page End: 2577
• Publication Date: 2018-06-19
• Subjects: natural hazards -- suspended sediment -- sediment load -- floods -- coastal watershed -- climate change -- extreme events; Geomorphology -- Periodicals; 551.4
• DOI: 10.1002/esp.4415
• Languages: English
• ISSNs: 0197-9337

Abstract

Abstract: Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10‐year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, these flows exported more than half as much sediment as had a 100‐year flood 35 years earlier, substantially enlarging the nearshore delta. Annual sediment load in 2017 was an order of magnitude greater than during an average‐rainfall year, and 500‐fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for several years. Future projections of megadroughts punctuated by major atmospheric‐river storm activity suggest that interannual sediment‐yield variations will become more extreme than today in the western USA, with potential consequences for coastal management, ecosystems, and water‐storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. Abstract : This study documents fluvial sedimentary and nearshore geomorphic response of a steep watershed to record seasonal rainfall in coastal California. We demonstrate a regime shift to greater and coarser sediment export resulting from modest storms that caused repeated 2–10‐year floods, a hydroclimatic situation likely to become part of the 'new normal' in future western US climate, alternating with drought conditions. This order‐of‐magnitude increase in fluvial sediment load was unrelated to the ENSO cycle, necessitating caution when interpreting marine sedimentary deposits to avoid misinterpreting past ENSO frequency.